1. Field of the Invention
This invention relates to a method of enlarging a design on a synthetic resin film having the design.
2. Description of the Related Art
A keytop 200 of the kind shown in FIGS. 7(A) and 7(B) has been developed as a keytop which is simple in structure and lends itself to a reduction in size and thickness [see the specification of Japanese Patent Application Laid-Open (KOKAI) NO. 6-44859]. The keytop 200 is formed by clamping a synthetic resin film 201 between upper and lower molds defining a cavity in the shape of a keytop, introducing a molten molding resin 211 under pressure from the side of the lower mold, thereby causing the synthetic resin film 201 to curvingly deform in the upward direction and form a curved portion 203 the shape of which is identical with that of the cavity in the upper mold, and simultaneously filling the recess on the lower side of the curved portion 203 with the molding resin 211.
The connection between the synthetic resin film 201 and the molding resin 211 is achieved by a bonding-agent layer 205, which is applied to the synthetic resin film 201 in advance by printing, before the molding resin 211 is introduced under pressure. By printing a desired design 207, which comprises a print layer, between the synthetic resin film 201 and the bonding-agent layer 205, the design 207 will appear on the top side of the keytop 200.
In the example of the prior art described above, the curved portion 203 of the synthetic resin film 201 is formed by stretching the synthetic resin film 201. Consequently, the design 207, which is printed before the stretching operation, is stretched as well. In other words, the originally printed design 207 is enlarged.
In the course of repeating a variety of experiments, the inventors have found that all points on the curved portion 203 of the synthetic resin film do not exhibit the same rate of enlargement. Rather, the rate of enlargement differs from point to point.
This means that however accurate the original design 207 printed on the synthetic resin film 201, the enlarged design 207 obtained after manufacture of the keytop 200 will exhibit some deformation.
If the keytop 200 is circular in shape, as in the illustrated example of the prior art, the deformation of the design 207 is not that great. However, if the keytop 200 is elliptical or rectangular in shape, the amount of deformation is larger.
When the keytop 200 is manufactured, the periphery of the region where the curved portion 203 of the synthetic resin film 201 is formed is clamped firmly by the upper and lower molds. However, the clamping force of the upper and lower molds along the outer circumference of the cavity in the upper mold differs depending upon the particular location owing to a disparity in the thickness of the synthetic resin film 201 or a variance in the degree of flatness and degree of smoothness of the upper and lower molds. As a consequence, there is the danger that the synthetic resin film situated at the outer circumferential portion of the cavity will be damaged when the synthetic resin film 201 is stretched and curved at deformation thereof. In addition, a weakly clamped portion of the synthetic resin film 201 is pulled into the curved portion 203 by a greater amount than a strongly clamped portion, as a result of which the synthetic resin film 201 is not deformed uniformly. Thus there is the danger that the deformation of the design 207 will take place with even less accuracy.
Furthermore, if the print layer is provided on the entire upper surface of the synthetic resin film 201 shown in FIGS. 7(A) and 7(B), damage to the synthetic resin film 201 does not occur by virtue of the elasticity and viscosity of the print layer. However, besides a variance in the print layer, a variance which depends upon the location of the synthetic resin film 201 pulled into the curved portion 203 occurs in the same manner and the design 207 undergoes deformation inaccurately. This is just as set forth above in the example of the prior art.